Unravelling the subgrain stabilities of microstructures across P91 steel weld joint under creep-fatigue interaction loading waveforms through EBSD and synchrotron based XRD studies

The aim of the present work is to be able to explain the subgrain stability/changes in the perspective of statistically stored dislocations(SSDs) and geometrically necessary dislocations (GNDs) and the resultant dislocation characters due to their interactions in the various microstructural zones of P91 steel weld joint (WJ) under the effects of different types of creep-fatigue interaction (CFI) loading waveforms. Towards this, five different combinations of simultaneous creep and fatigue loadings in the CFI waveforms and one low cycle fatigue loading (i.e. with no creep hold) were utilized for experimentation. The susceptibility of the subgrains to coarsening depend upon the microstructural zone in consideration (i.e. hard zones (weld metal and coarse grain heat affected zone) or soft zones (base metal, intercritical heat affected zone and fine grain heat affected zone) of P91 steel WJ), asymmetricity/symmetricity of the CFI waveforms applied (i.e. application of either tensile or compressive hold alone or both tensile and compressive holds) and the extents of creep/fatigue contributions (short or long holds) in the CFI loadings. Since the resultant subgrains formed in the various microstructural zones across the P91 steel WJ under different cyclic loading waveforms are the complex interplay of dislocation-dislocation interactions among the initial dislocations, production and annihilations, the net dislocation densities have been determined in terms of GNDs and SSDs. The outcome of the study was that, while GNDs may primarily influence the overall cyclic softening of the P91 steel weld joint, the SSDs determined the resultant substructure sizes. • SSDs are microstructural zone & waveform dependent; GNDs are always decreasing • Larger annihilations of GNDs in soft IC, FG and BM compared to hard CG/WM • Higher reversals promote annihilations whereas lower reversals promote jogs • Due to high initial SSDs stability of hard zone subgrains remain unaffected • GNDs and SSDs relates to cyclic softening and substructural coarsening respectively